CHAPTER III. INVESTIGATION OF THE CHARGE STORAGE
3.3 Results and Discussion
3.3.5 In-situ XANES spectroscopy of -MnO 2 nanosheet electrodes equilibrated in
Figure 69. Raman scattering spectra of (a) hollandite MnO2, (b) coronadite MnO2, (c) romanechite MnO2, (d) todorokite MnO2, (e) birnessite MnO2, (f) pyrolusite MnO2, and
(g) ramsdellite MnO2. From Julien, et al.205
3.3.5 In-situ XANES spectroscopy of -MnO2 nanosheet electrodes equilibrated in
Figure 70. (a) XANES spectra of reference materials MnO, Mn3O4, LiMn2O4 and MnO2. (b) Relationship between K-edge energy and Mn oxidation state (standards) for
determining oxidation state of Mn in the samples.
The pH = 2, 4 and 9 treated -MnO2 nanosheet on glassy carbon electrodes were studied via in-situ XANES measurements at fully discharged and fully charged states in two consecutive CV cycles, with the obtained spectra shown in Figure 71 (a, c, and e).
Each spectrum was obtained by averaging nine independently measured spectra at two different spots on the MnO2 nanosheet surface. The in-situ spectra showed that all MnO2
nanosheets exhibit same XANES features compared to the pH = 2 and 4 equilibrated MnO2 powders (data shown in Figure 48), indicating similar structural characteristics of Mn under different pH equilibrations and various applied potentials. However, a definitive shift of the absorption edge to lower energies has been observed for all the three samples when decreasing the applied potentials (charging the MnO2 electrodes).
The shifting of the absorption edge to a lower energy is related to a decrease in binding energy of the core electrons due to a decreasing oxidation state, thus representing the K+ intercalation Faradaic redox reaction as shown in Equation (9), as well as the formation of Mn3+. It is also observed that when re-discharging the electrode, the absorption edge of the XANES spectra moves back to almost the same position for all three samples, indicating high reversibility of the Faradaic redox reaction and K+ intercalation process.
In order to further investigate the effect of defects on the charge storage process, we have plotted the variation of the Mn oxidation state upon charging for all three samples, and compared the trend obtained from XANES data to the values expected from the CV loop, with the results shown in Figure 71 (b, d, and f) and Figure 72. The EC calculated data is plotted based on the assumption that the potassium ion insertion is accompanied with the reduction of one Mn ion, and the value is calculated from the specific capacitance (determined from the CV loop). First, it can be clearly seen that with decreasing the pH equilibration value, the change of Mn oxidation state obtained from XANES data becomes more pronounced, indicating that more Mn4+ has been reduced to Mn3+ with the presence of more surface Frenkel defects. This is likely because the defects can help promote the ion intercalation and charge transfer process, thus facilitating the Faradaic redox reactions and reduction of Mn. This is also consistent with our previous results shown in Chapter II and further confirms that the Mn surface Frenkel defects in δ- MnO2 nanosheets can help increase the alkali cation intercalation properties.25
.
Figure 71. In-situ XANES spectra and calculated Mn oxidation states obtained at fully discharged and fully charged states for (a, b) pH = 2, (c, d) pH = 4, and (e, f) pH = 9 equilibrated MnO2 nanosheet electrodes. Figures (b, d, and f) also show the comparison
Figure 72. Mn oxidation state change comparison upon charge/discharge of pH = 2, 4 and 9 equilibrated MnO2 nanosheet electrodes.
Further investigation of the pH = 9 treated electrodes shows that the oxidation state change obtained from XANES data (0.10 electrons) is close to that calculated from the specific capacitance (0.12 electrons). The difference between these two values might be attributed to the double-layer capacitance, since the reassembled nanosheets typically exhibit large surface area. However, when decreasing the pH equilibration value to 4 and 2, the discrepancy between measured data and theoretical line increases to 0.04 and 0.09 electrons, respectively. Thus, a smaller change of Mn oxidation states as compared to the CV loops determined values has been observed for the MnO2 nanosheets with higher defect content. Since lower pH equilibration leading to the formation of more surface Frenkel defects, the above results imply that the potassium ions can be inserted without reduction of Mn cations, assuming a large amount of Mn vacancies in the MnO2 nanosheets can serve as new intercalation sites for potassium ions without change of Mn oxidation state. The observed extra capacitance cannot be all attributed to the double- layer capacitance, since the three samples exhibit similar surface area.
Recently, it has been reported that the surface hydroxyl groups can serve as charge storage sites, which results in extra capacity without affecting the oxidation state of host cations.124,208,209
According to Cross et al,208 when the electrode is underwent cathodic electrochemical scan, the surface hydroxyl groups on the MnO2 can be polarized, and
they can attract a proton or metal ion from the electrolyte for charge balance. Thus, the surface hydroxyl groups can serve as extra charge storage sites. Their calculations based on the -MnO2 crystal structure and 0.8 V voltage window result a capacitance of ~2750 F/g, which shows the potential that the surface hydroxyl groups can greatly contribute to the overall capacitance. Direct observation of more surface hydroxyls on pH = 2 equilibrated MnO2 nanosheets has been obtained by de-convoluting the O 1s XPS spectra, with the results shown in Figure 73. Generally speaking, the peak located in between 529-530 eV is attributed to Mn-O-Mn (the lattice oxygen), and the peak located in between 530-531 eV is originated from Mn-OH (surface hydroxyls).210-212 The relative ratios of surface hydroxyls and lattice oxygen in both samples have been obtained through fitting the O 1s spectra, and the results indicate that the surface hydroxyl concentration in pH = 2 equilibrated MnO2 nanosheet floccs (43%) is much higher than that in the pH = 4 equilibrated sample (22%). Thus, we assume that hydroxyls or protonated oxygen sites around Mn vacancies in our defective MnO2 nanosheet system may also help accumulate the charges upon K+ ions intercalation without affecting the oxidation state of Mn, which may explain the observed extra capacitance. In general, the in-situ XANES studies revealed that although the Faradaic redox reaction is still the main charge storage mechanism, a large amount of potassium ions can still intercalate into the defective MnO2 nanosheets without reduction of Mn cations due to the presence of extra Mn vacancies that are surrounded by hydroxyls and protonated oxygen groups.